The present invention relates generally to double containment piping systems and, more particularly, to field-fabricated joints for such piping and a method of making such joints between adjacent pipe lengths and/or fittings.
Thermoplastic pipes are widely used in the chemical and petrochemical industries because of their low cost, light weight and ability to withstand prolonged exposure to chemicals which may have a deleterious affect on other, non-plastic piping. Because of the chemical resistance of thermoplastic pipes, they are also commonly used in the construction of double containment piping systems. In double containment piping systems, an inner carrier pipe which conveys the system fluid is surrounded, or enclosed by an outer containment pipe which collects any system fluids which may leak from the inner carrier pipe. The outer containment pipe serves as a barrier between the environment and the system fluids conveyed by the carrier pipe. Double containment piping systems are made tap of numerous pipe lengths and fittings which are interconnected by various joints. The joints of adjacent pipe sections need to have a structural integrity which is equal to or greater than that of the pipe sections themselves.
There are many joints known for double containment piping systems in the art. For example, U.S. Pat. No. 4,157,194 issued Jun. 5, 1979 and describes a thermoplastic double containment pipe system. The joints of this piping system require separately formed joint members which are inserted into opposing ends of the primary pipe portions and either rod welded or extrusion welded together.
U.S. Pat. No. 4,930,544, issued Jun. 5, 1990 and describes a restraining coupling which joins opposing end portions of thermoplastic double containment pipes together. This restraint coupling has a complex shape, thereby increasing its manufacturing cost, and may not be joined to the opposing pipe section by a welding sleeve, but rather requires rod or extrusion welding.
U.S. Pat. No. 5,087,308, which issued Feb. 11, 1992, describes a method of joining double containment pipes together wherein thermoplastic carrier and containment pipes are joined together by electric welding sleeves. The joint described in this patent between the outer containment pipes requires a spacer ring to complete a small gap between opposing end portions of the containment pipes prior to the welding sleeve being inserted in place. The gap filled by this ring is not large enough to permit access to the primary pipe welding sleeve for repair.
None of the above patents provide for a double containment piping joint for thermoplastic pipes in which the joint has improved structural characteristics.